W
w_tom
You have the frequency spectrum chart for lightning. Each
frequency is AC current. Each frequency contains some of the
energy from CG lightning.
How were radio transmitters created only from DC batteries?
A spark gap created AC from DC. Not very efficient. But like
lightning, the discharge through a non-linear medium (the
spark gap) puts energy into the AC regions - as demonstrated
by that frequency spectrum chart.
If lightning were only DC, then capacitors (ie. open
switches rated to exceed the breakdown voltage) would be
sufficient to block lightning. Lightning cannot be stopped.
Even if the breakdown voltage is not exceeded, still some
current passes through the switch due to AC components of that
lightning strike. If lightning were only DC, then long wires
to earth would easily ground lightning. Again, it is the AC
components of lightning that causes telcos to put their
switching computers up to 50 meters after the surge protector
AND put their surge protector right on earth ground. Wire
impedance of lightning puts significant energy in AC
components - as demonstrated by that frequency spectrum chart.
Either energy can be transferred by DC, or energy can be
transferred by AC. Clearly much of the energy from lightning
is found in AC (radio frequencies).
Wire has impedance. A sharp bend is the equivalent of
increasing that inductance (and therefore impedance) by
factors such as 6 times - a ballpark number. To 60 hz
electricity, this impedance is trivial and irrelevant. But
lightning is different electricity. We are talking about
10,000 amps with a rising edge of 8 microseconds. Anything
that increases wire inductance (such as splices, sharp bends,
routing through metallic conduit, etc) means a lightning
transient will seek alternative paths inside the building.
One trick in commercial radio stations to make lightning
seek earth ground is to feed the antenna through a coiled wire
or ferrite bead. That ferrite bead may be trivial to a radio
transmitter. But to lightning, that ferrite bead encourages
lightning to take earth ground at the antenna base rather than
find earth through the adjacent transmitter shed. Again, it
is the sharp rise time of that pulse combined with the massive
(and short) currents that make low impedance ground wire so
important to lightning protection. Generally increasing the
wire gauge provides little benefit. Decreasing wire length
(and therefore wire inductance) provides a much better
improvement.
To provide numbers as example: the 50 feet of 20 amp
electric wire may be less than 0.2 ohms resistance. That same
wire could be 120 ohms impedance to lightning. Voltage
difference be between a wall receptacle and earthed breaker
box if a plug-in protector were earthing a tiny 100 amp
surge? Less than 12,000 volts. That 0.2 ohms resistance is
not the problem. That 120 ohms impedance is just another
reasons why plug-in protectors are not properly earthed at
wall receptacles.
To reduce wire impedance, some installations use flat ribbon
wire instead of solid copper. However other problems such as
weather and corrosion must also be considered which is why
solid wire is often used for earthing. Military facilities
are suppose to inspect this earthing system every 5 years or
less. How often does the home owner do his inspection? Many
home owners don't even know an earth ground exists or is
necessary. Just another reason why we make compromises
between lower impedance wire and corrosion resistant
solutions.
Many reasons for keeping a lightning rod earth ground wire
outside the building involves factors beyond the scope of this
discussion. But one reason why: once inside the building,
then a destructive transient has a building is chock full of
conductors. The building concrete. The heating system.
Linoleum tile. Etc. The point is once lightning is inside
the building, then the building has too many conductive paths
to create destructive and induced transients. This is but
another reason why we earth transients before transients enter
the building.
Analysis and elimination of those so many conductive paths
inside a building is just too expensive and complicated.
Earth lightning outside the building and a majority of
destructive transients are eliminated. Keep lightning outside
the building so that protective circuits inside appliances are
not overwhelmed. Earth a transient through an adjacent plug-in
protector - even a trivial 100 amp transient and the protector
is something less than 12,000 volts relative to ground -
ineffective protection.
BTW, we earth to accomplish two goals. First we conduct
lightning to earth by the most conductive path possible. But
realities say we cannot do that well enough. So we attempt to
make earth beneath the building equipotential using concepts
such as single point ground, Ufer or halo grounds, etc.
However we can never make earth equipotential enough. So we
make the earthing connection more conductive.
Effective protection costs so little and is so much more
effective when the system is planned for and installed as
footing are poured. Add on solutions, as is standard in most
construction today, tends to be either more expensive or less
effective. One man's experience:
http://scott-inc.com/html/ufer.htm
frequency is AC current. Each frequency contains some of the
energy from CG lightning.
How were radio transmitters created only from DC batteries?
A spark gap created AC from DC. Not very efficient. But like
lightning, the discharge through a non-linear medium (the
spark gap) puts energy into the AC regions - as demonstrated
by that frequency spectrum chart.
If lightning were only DC, then capacitors (ie. open
switches rated to exceed the breakdown voltage) would be
sufficient to block lightning. Lightning cannot be stopped.
Even if the breakdown voltage is not exceeded, still some
current passes through the switch due to AC components of that
lightning strike. If lightning were only DC, then long wires
to earth would easily ground lightning. Again, it is the AC
components of lightning that causes telcos to put their
switching computers up to 50 meters after the surge protector
AND put their surge protector right on earth ground. Wire
impedance of lightning puts significant energy in AC
components - as demonstrated by that frequency spectrum chart.
Either energy can be transferred by DC, or energy can be
transferred by AC. Clearly much of the energy from lightning
is found in AC (radio frequencies).
Wire has impedance. A sharp bend is the equivalent of
increasing that inductance (and therefore impedance) by
factors such as 6 times - a ballpark number. To 60 hz
electricity, this impedance is trivial and irrelevant. But
lightning is different electricity. We are talking about
10,000 amps with a rising edge of 8 microseconds. Anything
that increases wire inductance (such as splices, sharp bends,
routing through metallic conduit, etc) means a lightning
transient will seek alternative paths inside the building.
One trick in commercial radio stations to make lightning
seek earth ground is to feed the antenna through a coiled wire
or ferrite bead. That ferrite bead may be trivial to a radio
transmitter. But to lightning, that ferrite bead encourages
lightning to take earth ground at the antenna base rather than
find earth through the adjacent transmitter shed. Again, it
is the sharp rise time of that pulse combined with the massive
(and short) currents that make low impedance ground wire so
important to lightning protection. Generally increasing the
wire gauge provides little benefit. Decreasing wire length
(and therefore wire inductance) provides a much better
improvement.
To provide numbers as example: the 50 feet of 20 amp
electric wire may be less than 0.2 ohms resistance. That same
wire could be 120 ohms impedance to lightning. Voltage
difference be between a wall receptacle and earthed breaker
box if a plug-in protector were earthing a tiny 100 amp
surge? Less than 12,000 volts. That 0.2 ohms resistance is
not the problem. That 120 ohms impedance is just another
reasons why plug-in protectors are not properly earthed at
wall receptacles.
To reduce wire impedance, some installations use flat ribbon
wire instead of solid copper. However other problems such as
weather and corrosion must also be considered which is why
solid wire is often used for earthing. Military facilities
are suppose to inspect this earthing system every 5 years or
less. How often does the home owner do his inspection? Many
home owners don't even know an earth ground exists or is
necessary. Just another reason why we make compromises
between lower impedance wire and corrosion resistant
solutions.
Many reasons for keeping a lightning rod earth ground wire
outside the building involves factors beyond the scope of this
discussion. But one reason why: once inside the building,
then a destructive transient has a building is chock full of
conductors. The building concrete. The heating system.
Linoleum tile. Etc. The point is once lightning is inside
the building, then the building has too many conductive paths
to create destructive and induced transients. This is but
another reason why we earth transients before transients enter
the building.
Analysis and elimination of those so many conductive paths
inside a building is just too expensive and complicated.
Earth lightning outside the building and a majority of
destructive transients are eliminated. Keep lightning outside
the building so that protective circuits inside appliances are
not overwhelmed. Earth a transient through an adjacent plug-in
protector - even a trivial 100 amp transient and the protector
is something less than 12,000 volts relative to ground -
ineffective protection.
BTW, we earth to accomplish two goals. First we conduct
lightning to earth by the most conductive path possible. But
realities say we cannot do that well enough. So we attempt to
make earth beneath the building equipotential using concepts
such as single point ground, Ufer or halo grounds, etc.
However we can never make earth equipotential enough. So we
make the earthing connection more conductive.
Effective protection costs so little and is so much more
effective when the system is planned for and installed as
footing are poured. Add on solutions, as is standard in most
construction today, tends to be either more expensive or less
effective. One man's experience:
http://scott-inc.com/html/ufer.htm
